Ousted in a general election last year after a corruption scandal for which he is on trial, the former prime minister says the accusations by a death-row police officer that he was behind the slaying of 28-year-old Altantuya Shaariibuu is a political ploy to jail him.
Policeman Azilah Hadri and another officer, who were serving on Najib's security detail at the time of the murder, were sentenced to death in 2015 for the killing of Shaariibuu.
Najib was Malaysia's deputy prime minister and held the defense portfolio when Shaariibuu was shot dead and her body was blown up by military-grade explosives in a forest near the capital, Kuala Lumpur.
"In Allah’s noble house, and on this thankful Friday, I declare and swear ... I have never instructed any individual to murder a woman of Mongolian nationality named Altantuya Shaariibuu," Najib read from a sheet of paper at a mosque in central Kuala Lumpur, his wife and daughter by his side.
"And I have never met and did not know the dead at all. If I lie, then the curse of Allah is upon me and if I am true, then those who slander me and do not repent will be cursed by Allah," drawing cheers from nearly 1,000 supporters who witnessed his oath.
Najib has never faced any charges in connection with the woman's murder. He is however, facing criminal charges in connection with a multibillion-dollar scandal at state fund 1Malaysia Development Bhd (1MDB).
He has pleaded not guilty to the charges of criminal breach of trust, money laundering and abuse of power.
It has never been established who was behind the two policemen convicted of killing Shaariibuu.
But Najib's former bodyguard Azilah, in a court filing this month seeking to set aside his conviction, said Najib had ordered him to "arrest and destroy" Shaariibuu, who the former premier had allegedly described as a foreign spy, news portal Malaysiakini reported on Monday.
Malaysia's Federal Court will hear Azilah's application on April 20.
Civil society groups have alleged Shaariibuu's murder was linked to her role as an interpreter for Abdul Razak Baginda, a former associate of Najib, in Malaysia's purchase of two submarines from French shipbuilding giant DCNS in 2002.
Abdul Razak, who said he had an affair with Shaariibuu, was in 2008 acquitted of abetting the murder.Azilah's co-accused, Sirul Azhar Uma, fled to Australia shortly before his sentence was handed down.